This invention deals with illumination delivery systems, in particular for application in a macroscopic defect inspection system for patterned wafers.
Macroscopic defect inspection systems employed in semiconductor processing have the capability of discerning defect anomalies from a patterned background. Generally, repetitive structures are compared, using dark field scattered profiles or bright field images, between a minimum of three die to locate defects. Detectable defect sizes using this type of system range from the order of tens of microns to whole-wafer sized defects, up to 200 or 300 mm. Spatial and angular uniformity of the illumination over the full field of view are crucial to achieving reproducibly accurate results, and to avoid changes in the diffraction pattern from the wafer.
Tool architectures can be comprised of a zero-dimensional light source (a point source), a one-dimensional light source (a line source), or a two-dimensional light source (area source). In the first case, the point source and the substrate move relative to each other in both x and y directions, or radially and angularly. In the second case, the line source and the substrate typically move relatively in one direction. In the case of a full wafer area source, the substrate and source do not move relative to each other. If the area source is less than the area of the wafer, then movement of the source and substrate are as in the point source case.
The Viper inspection tool from KLA-Tencor is a macroscopic defect inspection system which employs a one-dimensional (i.e. line) light source, which is scanned across the wafer surface in one direction only, perpendicular to the line of the light source. The Viper-2401 system, which is designed for inspection of 200 mm diameter wafers, is described in U.S. Pat. No. 5,917,588 by Addiego, which is hereby incorporated by reference in its entirety.
In the Viper tool architecture, the substrate of interest is scanned by a line illumination source and the resulting dark field scatter profile or bright field image is collected by a linear array camera. The Viper 2401 system utilizes a cylindrically shaped lamp such as a fluorescent tube, illustrated in FIG. 1 in dark-field configuration. Lamp tube 2 functions as a line illuminator. Outgoing light rays 4 emerge from line aperture 6 to shine on wafer surface 8. Scattered rays 10 are focused by imaging lens optics 12 onto linear sensor array 13.
Improvements in the light source, including greater spatial and angular uniformity, and increasing intensity, are important factors in the development of improved defect detection systems, particularly as wafer size increases, for example up to 300 mm diameter. Tube lamps may not provide uniform illumination profiles (including intensity as a function of wavelength, and direction) at the substrate plane over the length of interest, nor may they be able to sustain the same constant profile as a function of length over time. A lamp that can maintain a constant illumination profile over time may be usable with suitable spatial corrections applied; however, a lamp whose illumination profile changes with age is more difficult to use successfully since spatial corrections would need to be adjusted as well.
An inherent problem with extended light sources such as fluorescent lamps is the difficulty in using a closed loop control system to stabilize the illumination profile as a function of time, since sensing the light output at any point on the lamp does not necessarily reflect the light output changes elsewhere.
Another problem with the use of fluorescent lamps in defect detection tools, particularly as the diameter of the wafers and therefore the necessary length of the illumination source, increases, is the desirability of increased intensity, for several reasons. First, if pixel size were maintained fixed as wafer size increased, and therefore the number of pixels across the wafer increased, a large increase in complexity of data analysis and slower throughput would result. It is therefore desirable to increase the pixel size in proportion to wafer size increase, and thereby maintain the total number of pixels fixed. By doing so, the spatial sampling frequency is lowered which can cause problems. If the sampling frequency is lowered below the Nyquist value, particularly at feature edges where the spatial feature frequency is highest, aliasing effects may occur. In order to avoid this, the numerical aperture of the collection lenses must be lowered so as to reduce the high frequency component of the signal. The lower numerical aperture requires higher intensity light. Secondly, higher intensity light will also increase the signal to noise ratio, yielding a more precise and accurate image within each pixel. Thirdly, increased intensity is necessary to enable polarization or color imaging, both of which involve filtering out a large portion of the light. The intensity of fluorescent lamps is insufficient for these purposes. Additionally, there is a large amount of light intensity wasted from a fluorescent bulb source due to lack of angular control.
The use of a high intensity point light source would solve the problem of insufficient intensity, and would additionally make feasible a closed-loop control approach to insure stability over time. However, in order to be utilized in a line-scan application such as the Viper tool, the light from a point source would be required to be reshaped into a uniform line.
It is therefore an object of this invention to provide an illumination delivery system including a long linear light source having intense, spatially and temporally uniform output across its length.
It is a further object of this invention to provide an illumination delivery system including a long linear light source which is adaptable to closed loop control.
It is a further object of this invention to provide an illumination delivery system including a long linear light source which has precise control of incident angle and of incident angular spread.
These objects are met by directing the output of a point light source into a shaped fiber optic bundle, then afterwards mixing, diffusing, and focusing the fiber optic output to provide an intense, uniform linear light source with well defined incident angle.